ci: install pnpm via npm (gongfoo rust image lacks corepack)

The build job failed with `corepack: command not found`: Fedora's nodejs
package (gongfoo runner-rust image) ships node + npm but not corepack. The
job runs as root, so install pnpm globally via npm instead. Longer term,
bake pnpm into the runner image to drop this step.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01X7zF7Kf4JqDwa6M8Qgge9M

.gitea/workflows/refresh.yml

@@ -38,8 +38,8 @@ jobs:
       - uses: actions/checkout@v4
       - name: Build web (vite client + prerender)
         run: |
-          corepack enable
-          corepack prepare pnpm@10 --activate
+          # gongfoo rust image has node + npm but not corepack; job runs as root.
+          npm install -g pnpm@10
           pnpm --dir ui install --frozen-lockfile
           pnpm --dir ui run build
       - uses: actions/upload-artifact@v3